1. Field of the Invention
The present invention relates to an extrusion technique for thermoplastic deformable plastics, especially the manufacture of plastic plates and foils. It provides automatic regulation of the roller conveyor so as to guide the extruded plastic plate or foil through the center of the extrusion device after the plate or foil leaves the roller calender.
2. Description of the Related Art
Extrusion devices for thermoplastic plastics are known. The melted polymer exits the extruder over a wide slit nozzle and runs through a roller calender, whereby the thickness of the extruded plastic is established. The plastic may be in the form of plates, foils or similar bodies, hereinafter simply referred to as plates. The plastic plate leaves the roller calender in a considerably rigid condition. It runs horizontally over a gravity-roller conveyor for further cooling and is then taken over by two haul-off rolls which control the running direction of the plastic plate, where the running direction is the direction of movement since the plastic: plate left the calender.
If the plastic plate is not symmetric to the geometric center of the device, but off-set to the right or left, there may result disturbances in subsequent devices such as edge cutting saws, transverse separating saws, staplers or spoolers. The resulting plate section would be, for example, not right-angled and thereby would have to be discarded. Therefore, the guiding of the plastic plate through the horizontal center of the device must be constantly supervised.
If the plastic plate deviates from the centered course, the haul-off rolls must be opened so that it can be pulled into the correct position. Another possibility is to regulate the angle of the haul-off rolls relative to the conveyor direction. For the exact in-regulation or after-regulation, a mechanical controller must be built in.
Also known is so-called rotation frame regulation (Erhard and Leimer Company, Augsburg, Austria). It guides foils by oscillating the rotation frame. A changing rolling-up of the foil can be achieved by running oscillation of the rotation frame but is not appropriate for guiding plastic plates.
Different measuring sensors are known for measuring the plastic plate width, including thermographic or mechanical measuring sensors, and infrared measuring sensors which scan in a direction transverse to the running direction to determine the plate edges (Octagon Company, W urzburg, Germany). While these techniques can detect the width of the plastic plate, they will not be accurate unless the plate is moving through the center of the device. Disturbances caused by sideways deviation of the plastic plate must therefore be eliminated.